Modulation of autonomic neuroeffector transmission by nitric oxide in guinea pig ileum

The nitric oxide donor SNAP-induced amino acid neurotransmitter release in cortical neurons. Effects of blockers of voltage-dependent sodium and calcium channels

BACKGROUND

The discovery that nitric oxide (NO) functions as a signalling molecule in the nervous system has radically changed the concept of neuronal communication. NO induces the release of amino acid neurotransmitters but the underlying mechanisms remain to be elucidated.

FINDINGS

The aim of this work was to study the effect of NO on amino acid neurotransmitter release (Asp, Glu, Gly and GABA) in cortical neurons as well as the mechanism underlying the release of these neurotransmitters. Cortical neurons were stimulated with SNAP, a NO donor, and the release of different amino acid neurotransmitters was measured by HPLC. The involvement of voltage dependent Na+ and Ca2+ channels as well as cGMP in its mechanism of action was evaluated.

CONCLUSIONS

Our results indicate that NO induces release of aspartate, glutamate, glycine and GABA in cortical neurons and that this release is inhibited by ODQ, an inhibitor of soluble guanylate cyclase. Thus, the NO effect on amino acid neurotransmission could be mediated by cGMP formation in cortical neurons. Our data also demonstrate that the Na+ and Ca2+ voltage- dependent calcium channels are involved in the NO effects on cortical neurons.

L-citrulline inhibits [3H]acetylcholine release from rat motor nerve terminals by increasing adenosine outflow and activation of A1 receptors

BACKGROUND AND PURPOSE

Nitric oxide (NO) production and depression of neuromuscular transmission are closely related, but little is known about the role of L-citrulline, a co-product of NO biosynthesis, on neurotransmitter release.

EXPERIMENTAL APPROACH

Muscle tension recordings and outflow experiments were performed on rat phrenic nerve-hemidiaphragm preparations stimulated electrically.

KEY RESULTS

L-citrulline concentration-dependently inhibited evoked [(3)H]ACh release from motor nerve terminals and depressed nerve-evoked muscle contractions. The NO synthase (NOS) substrate, L-arginine, and the NO donor, 3-morpholinosydnonimine chloride (SIN-1), also inhibited [(3)H]ACh release with a potency order of SIN-1>L-arginine>L-citrulline. Co-application of L-citrulline and SIN-1 caused additive effects. NOS inactivation with N(omega)-nitro-L-arginine prevented L-arginine inhibition, but not that of L-citrulline. The NO scavenger, haemoglobin, abolished inhibition of [(3)H]ACh release caused by SIN-1, but not that caused by L-arginine. Inactivation of guanylyl cyclase with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) fully blocked SIN-1 inhibition, but only partially attenuated the effects of L-arginine. Reduction of extracellular adenosine accumulation with adenosine deaminase or with the nucleoside transport inhibitor, S-(p-nitrobenzyl)-6-thioinosine, attenuated the effects of L-arginine and L-citrulline, while not affecting inhibition by SIN-1. Similar results were obtained with the selective adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine. L-citrulline increased the resting extracellular concentration of adenosine, without changing that of the adenine nucleotides.

CONCLUSIONS AND IMPLICATIONS

NOS catalyses the formation of two neuronally active products, NO and L-citrulline. While, NO may directly reduce transmitter release through stimulation of soluble guanylyl cyclase, the inhibitory action of L-citrulline may be indirect through increasing adenosine outflow and subsequently activating inhibitory A(1) receptors.

Altered tachykinergic influence on gastric mechanical activity in mdx mice

This study investigated whether alterations in gastric activity in dystrophic mdx mouse can be attributed to dysfunctions of tachykinins. Endoluminal pressure was recorded and the expression of neuronal nitric oxide synthase (nNOS), NK1 and NK2 neurokinin receptors was investigated by immunohistochemistry. SR48968, NK2 receptor antagonist, but not SR140333, NK1 receptor antagonist, decreased the tone only in mdx gastric preparations. In the presence of N(omega)-nitro-l-arginine methyl ester (l-NAME), inhibitor of NOS, SR48968 reduced the tone also in normal stomach. [Sar(9), Met(O(2))(11)]-SP, agonist of NK1 receptors, caused tetrodotoxin-sensitive relaxations, antagonized by SR140333 or l-NAME, with no difference in the potency or efficacy between normal and mdx preparations. [beta-Ala(8)]-NKA(4-10), an NK2 receptor agonist, induced SR48968-sensitive contractions in both types of preparations, although the maximal response of mdx tissues was significantly lower than normal preparations. Immunohistochemistry demonstrated a consistent reduction of nNOS and NK2 receptor expression in mdx stomach smooth muscle cells and no change in nNOS and NK1 receptor expression in neurones. In conclusion, in mdx stomach the activation of NK2 receptors plays a role in the development of the tone, associated with a reduced NO production by muscular nNOS. The hypo-responsiveness to NK2 receptors could depend on the reduced expression of these receptors.

Glutamate stimulation of acetylcholine release from myenteric plexus is mediated by endogenous nitric oxide

Glutamate was found to be an excitatory neurotransmitter in the enteric nervous system. Although several lines of evidence indicate a role of glutamate in the regulation of gut motility and secretion the physiological significance of glutamatergic transmission is not clear yet. We studied the effect of glutamate on [3H]acetylcholine release and nicotinamide adenine dinucleotide phosphate-diaphorase staining in longitudinal muscle strips with attached myenteric plexus of guinea pig ileum. L-glutamate (100 microM) significantly enhanced both the evoked [3H]acetylcholine release and the optical density of nicotinamide adenine dinucleotide phosphate-diaphorase positive neurones, i.e. the intensity of staining. The non-competitive N-methyl-D-aspartate receptor antagonist MK-801 (3 microM) abolished the stimulatory effect of L-glutamate on acetylcholine efflux. Similarly, the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (100 microM) significantly reduced the effect of L-glutamate on [3H]acetylcholine release and nicotinamide adenine dinucleotide phosphate-diaphorase staining. Our data suggest that endogenous nitric oxide seems to mediate the stimulatory effect of glutamate on acetylcholine release from guinea pig myenteric neurons.

Inhibitory effect of sildenafil on rat duodenal contractility In vitro: Putative cGMP involvement

1. Sildenafil citrate (Viagratrade mark; Pfizer, Sandwich, Kent, UK), a phosphodiesterase 5 inhibitor, rises cGMP levels in smooth muscle cells. It relaxes both vascular and visceral smooth muscle. In order to assess the intestinal effects of sildenafil, we decided to investigate its actions on rat duodenal motor activity in vitro. 2. In isolated duodenal segments maintained in Tyrode's solution, sildenafil exhibited a concentration-dependent antispasmodic effect on acetylcholine (ACh)-induced phasic contractions, with an IC50 value of 26.7 micromol/L (95% confidence interval (CI) 2.0-55.3 micromol/L). 3. Sildenafil also relaxed the carbamylcholine (CCh)-induced sustained contraction with an IC(50) of 16.2 micromol/L (95% CI 9.5-27.6 micromol/L). Sildenafil produced significant additional relaxation of 25.2 +/- 8.1% of the CCh-induced contraction, beyond basal tone. 4. Sildenafil reduced the amplitude of spontaneous duodenal contractions with an EC50 of 9.6 micromol/L (95% CI 5.7-16.2 micromol/L). This effect was significantly more potent than the effects of zaprinast and papaverine, which also reduced duodenal contractions with EC50 values of 91.6 micromol/L (95% CI 46.0-182.2 micromol/L) and 78.5 micromol/L (95% CI 37.1-166.3 micromol/L), respectively. 5. In preparations treated previously with methylene blue (10 micromol/L) or 1H-[1,2,4]oxadiazolo(4,3-a)quinoxalin-1-one (ODQ; 10 micromol/L), the EC50 values for the sildenafil effect were significantly increased to 39.0 micromol/L (95% CI 23.9-63.4 micromol/L) and 43.8 micromol/L (95% CI 24.5-78.3 micromol/L), respectively. These values were significantly greater than those obtained with sildenafil alone. 6. In conclusion, sildenafil has myorelaxant and antispasmodic effects on rat duodenal segments in vitro. The contractile inhibitory effect of sildenafil on rat isolated duodenum is probably due to intracellular cGMP accumulation as a result of its decreased degradation.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

Upregulation of muscarinic receptors by long-term nitric oxide inhibition in the rat ileum

1. The aim of the present study was to examine the effects of long-term nitric oxide (NO) blockade on contractions of the rat ileum induced by muscarinic agonists. 2. Male Wistar rats received the NO synthesis inhibitor NG-nitro-l-arginine methyl ester (l-NAME; 20 mg/rat per day) in drinking water for 7, 15, 30 and 60 days. Concentration-responses curves to methacholine and carbachol were obtained and pEC50 values were calculated. Saturation binding assays were performed in membranes prepared from rat ileum after 60 days of l-NAME treatment and the dissociation constant (KD) and maximal number of binding sites (Bmax) were determined by Scatchard analysis. 3. The NO synthase activity of the ileum was markedly reduced in all l-NAME-treated groups. At 60 days after l-NAME treatment, a significant increase in the potency of methacholine (fourfold) and carbachol (threefold) was observed. In binding studies, we found a significant increase in Bmax for [3H]-quinuclidinyl benzilate of approximately 57% in the l-NAME treated group without any significant change in KD values. The contractile response to methacholine was not modified by the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (3 micro mol/L). No morphological alterations in the rat ileum were observed in l-NAME-treated rats. 4. Our findings suggest that treatment with l-NAME for 60 days induces a marked increase in the potency of methacholine and carbachol, as well as an increase in receptor number in the rat ileum.

The effects of exogenous nitric oxide on the function of neuromuscular synapses

Extracellular recording experiments using neuromuscular skin/chest muscle preparations from lake frogs were performed at low extracellular Ca2+ ion concentrations to study the effect of L-arginine (the substrate for nitric oxide synthesis) and N(G)-nitro-L-arginine methyl ester (a blocker of NO synthase) on the parameters of evoked transmitter secretion and ion currents in motor nerve endings. L-arginine at a concentration of 100 microM decreased the amplitude of endplate currents as well as their quantum composition, and also increased the amplitude of the third phase of the evoked nerve ending response, which reflects the kinetics of potassium influx currents. N(G)-nitro-L-arginine methyl ester at a condition of 100 microM led to increases in the amplitude and quantum composition of endplate currents and decreased the amplitude of the third phase of the evoked nerve ending response. It is suggested that endogenous nitric oxide is produced in frog neuromuscular synapses, which in normal conditions suppresses transmitter secretion and modulates the function of potassium channels in the nerve ending.

Functional coupling between nitric oxide synthesis and VIP release within enteric nerve terminals of the rat: involvement of protein kinase G and phosphodiesterase 5

1. The subcellular mechanisms involved in the effect of nitric oxide (NO) on the release of vasoactive intestinal polypeptide (VIP) were examined in synaptosomes isolated from rat small intestine. 2. VIP release was stimulated by the NO donor SNAP (10(-7)-10(-4) M) in an oxyhaemoglobin-sensitive manner. The presence of the guanylate cyclase inhibitor ODQ (10(-5) M), or inhibition of protein kinase G (PKG) by KT 5823 (3 x 10(-6) M) or Rp-8Br-PET-cGMPS (5 x 10(-7) M), antagonized the SNAP-induced VIP release, suggesting a regulatory role of PKG, confirming previously published data from enteric ganglia. This finding was further supported by the fact that direct PKG activation by the stable cGMP analogue 8-pCPT-cGMP stimulated VIP secretion to the same extent as SNAP. 3. Basal VIP secretion was enhanced in the presence of zaprinast, an inhibitor of cGMP-dependent phosphodiesterase 5 (PDE 5), suggesting a functional role of PDE 5 in NO-cGMP signalling. Supportive evidence for this finding was obtained by demonstration of the presence of PDE 5 using RT-PCR. 4. Stimulation of endogenous NO production by L-arginine was also effective in releasing VIP. The effect was abolished in the presence of KT 5823, but was insensitive to oxyhaemoglobin (10(-3) M), suggesting that an interaction between NO and VIP is likely to occur within the same nerve terminal rather than between terminals. 5. NO synthesis was not affected by VIP (10(-8)-10(-5) M), suggesting that there is no feedback regulation between the NO and the VIP pathways. 6. These findings support the notion that an anatomical and functional interrelationship exists between NO and VIP in enteric nerve terminals and that complex signalling mechanisms involving PKG and PDE 5 contribute to NO-induced VIP release.

Nitrergic and purinergic regulation of the rat pylorus

The role of nitric oxide (NO) and ATP in the regulation of nonadrenergic, noncholinergic (NANC) inhibitory transmission in the pylorus remains unclear. In the presence of atropine and guanethidine, electric field stimulation induced NANC relaxations in a frequency-dependent manner (1-20 Hz) in the rat pylorus. NANC relaxations were significantly inhibited by N(G)-nitro-L-arginine methyl ester (L-NAME; 10(-4) M). P(2X) purinoceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 3 x 10(-5) M) and P(2Y) purinoceptor antagonist reactive blue 2 (2 x 10(-5) M) had no effect on NANC relaxations. However, the combined administration of L-NAME and PPADS, but not reactive blue 2, evoked greater inhibitory effects on NANC relaxation than that evoked by L-NAME alone. alpha-Chymotrypsin and vasoactive intestinal polypeptide antagonist did not affect NANC relaxations. ATP (10(-5)-10(-3) M) and P(2X) purinoceptor agonist alpha, beta-methyleneadenosine 5'-triphosphate (10(-7)-10(-5) M), but not P(2Y) purinoceptor agonist 2-methylthioadenosine 5'-triphosphate (10(-7)-10(-5) M), induced muscle relaxations in a dose-dependent manner, and relaxations were significantly reduced by PPADS and unaffected by TTX. These studies suggest that NO and ATP act in concert to mediate NANC relaxation of the rat pylorus. ATP-induced relaxation appears to be mediated by P(2X) purinoceptors located on smooth muscle cells.

Nitric oxide-containing neurons in the bovine gut, with special reference to their relationship with VIP and galanin

The presence and distribution of nicotinamide dinucleotide phosphate diaphorase (NADPH-d)-containing neurons have been studied by means of NADPH-d histochemistry in different regions of the adult cow gut, from the esophagus to the rectum. NADPH-d and nitric oxide synthase (NOS) were constantly recognized to be colocalized in the same neuron. The colocalization of vasoactive intestinal polypeptide (VIP) and galanin in such nitrergic neurons was also studied by means of combined histochemical and immunofluorescence techniques. NADPH-d-positive neurons were present along the myenteric plexus of the entire gut, and in the submucous plexus from the abomasum to the rectum. Notably, they formed two types of nerve networks in the submucous connective tissue of the jejunum-ileum. NADPH-d-positive innervation of the muscle layers occurred throughout the tract, and sometimes a clear correspondence was noted between the number of reactive fibres and the thickness of the muscle. Nitrergic fibres also occurred in the mucosa and often were in relation to glands and blood vessels. The nitrergic neurons varied in size, shape, and intensity of staining, and often their terminals were seen to surround unstained perikarya. Various types of neurons were recognized on the basis of their chemical content; one of them contained galanin, VIP and NOS simultaneously. The present results suggest that the nitrergic neurons of the bovine gastrointestinal tract play roles presumably for controlling the motility of the gut and the conduction of interneuronal impulses.

Regional regulation of choroidal blood flow by autonomic innervation in the rat

Regional influences of parasympathetic and sympathetic innervation on choroidal blood flow were investigated in anesthetized rats. Parasympathetic pterygopalatine neurons were activated by electrically stimulating the superior salivatory nucleus, whereas sympathetic neurons were activated by cervical sympathetic trunk stimulation and uveal blood flow was measured by laser Doppler flowmetry. Parasympathetic stimulation increased flux in the anterior choroid and nasal vortex veins but not in the posterior choroid. Vasodilation was blocked completely by the neuronal nitric oxide synthase inhibitor 1-(2-trifluoromethylphenyl)imidazole but was unaffected by atropine. Sympathetic stimulation decreased flux in all regions, and this was blocked by prazosin. Parasympathetic stimulation did not affect vasoconstrictor responses to sympathetic stimulation in the posterior choroid but attenuated the decrease in blood flow through the anterior choroid and vortex veins via a nitrergic mechanism. We conclude that sympathetic alpha-noradrenergic vasoconstriction occurs throughout the choroid, whereas parasympathetic nitrergic vasodilation plays a selective role in modulating blood flow in anterior tissues of the eye.

Involvement of nitric oxide in parasympathetic and antidromic vasodilatations in cat lower lip

The involvement of nitric oxide (NO) in the lower lip vasodilatations mediated via parasympathetic and antidromic mechanisms was examined in alpha-chloralose/urethane-anesthetized cats, with the two types of blood flow responses being recorded separately (by laser Doppler flowmeter) from the two sides of the lower lip. The central cut end of the lingual nerve (LN) or the peripheral cut end of the inferior alveolar nerve (IAN) was electrically stimulated to elicit parasympathetic or antidromic vasodilatation, respectively, in the lower lip. N(G)-nitro-L-arginine methyl ester (L-NAME), but not N(G)-nitro-D-arginine methyl ester (D-NAME) (each at 30 mg/kg), markedly reduced the increases in lip blood flow evoked by stimulation, the reduction being to a similar degree irrespective of whether LN or IAN was stimulated. Pretreatment with L-arginine did not prevent the L-NAME-induced attenuation of either type of vasodilatation. In conclusion, these results suggest that synthesized NO may have a common site of action in antidromic and parasympathetic vasodilator pathways to the cat lower lip.

Prokinetic effect of black tea on gastrointestinal motility

The gastrokinetic effects of hot water extract of black tea [Camellia sinensis, (L) O. Kuntze (Theaceae)] on gastrointestinal motility were studied both in vivo and in vitro. The extract significantly accelerated the gastrointestinal transit (GIT) in vivo in mice. These facilitatory effect was reduced after pretreatment with atropine, hemicholinium-3, morphine, indomethacin, McN-A-343 and L-arginine. In guinea pig ileum, the extract facilitated the peristaltic reflex in response to pressures in normal preparation. The black tea extract and L-NMMA (nitric oxide synthase inhibitor) significantly reduced the electrical field stimulated nonadrenergic, noncholinergic (NANC) relaxation of isolated rat fundal strips. The extract markedly enhanced the tonic ('hump') responses to transmural stimulation in longitudinal muscle of guinea pig ileum which was unaltered in the presence of atropine. These findings suggest a cholinergic involvement and a partial role of prostaglandin and nitric oxide in the mechanism of action of black tea extract on gastrointestinal motility. To determine the effective constituents in black tea responsible for this activity, the effect of black tea polyphenols on GIT were also studied. Thearubigin fraction (but not theaflavin) accelerated GIT significantly which suggests its involvement in the prokinetic effect of black tea.

Cyclic GMP-associated apamin-sensitive nitrergic slow inhibitory junction potential in the hamster ileum

1. The mediators of non-adrenergic, non-cholinergic (NANC) inhibitory junction potentials (i.j.ps) in the circular smooth muscle cells of the hamster ileum were studied. 2. Electrical field stimulation (EFS; 0.5 ms duration, 15 V) of the intramural nerves with a train of five pulses at 20 Hz evoked a rapidly developing hyperpolarization (fast i.j.p.) followed by a sustained hyperpolarization (slow i.j.p.). 3. NG-nitro-L-arginine methyl ester (L-NAME; 50 - 200 microM) and NG-nitro-L-arginine (L-NNA; 50 - 200 microM), NO synthase inhibitors, inhibited or abolished the EFS-induced fast and slow NANC i.j.ps. The effects of these NO synthase inhibitors were reversed by L-arginine (5 mM) but not by D-arginine (5 mM). 4. Exogenously applied nitric oxide (NO; 1 - 100 microM) induced concentration-dependent hyperpolarizations. 5. Oxyhaemoglobin (5 - 50 microM), NO scavenger, inhibited only the slow i.j.p., and the NO-induced hyperpolarization. 6. 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxaline-1-one (ODQ; 10 microM) and cystamine (10 mM), guanylate cyclase inhibitors, inhibited only the slow i.j.p. Zaprinast (100 microM), a phosphodiesterase type V inhibitor, enhanced the amplitude and duration of the slow i.j.p. 7. Apamin (100 nM), a small conductance Ca2+-activated K+ channel blocker, inhibited only the slow i.j.p., and NO-induced hyperpolarization. A high concentration of 8-bromoguanosine 3':5'-cyclic monophosphate (8-bromo-cGMP; 1 mM)-induced membrane hyperpolarization which was blocked by apamin. 8. These results suggest that NO, or a related compound, may be the inhibitory transmitter underlying the apamin-sensitive NANC slow i.j.p. and cyclic GMP mediates the slow i. j.p. in the hamster ileum. It is also likely that NO, without involvement of guanylate cyclase is associated with the fast i.j.p.

Hemolink-induced effects on intestinal motor function and attenuation of these effects by selected agents

Hemolink, an oxidized, ring-opened raffinose-crosslinked hemoglobin-based oxygen carrier produced by Hemosol Inc., stimulates esophageal peristalsis, possibly by interference with neural NO-mediated effects. The effects of Hemolink on jejunal tone and contractions, arterial pressure and heart rate were measured in anesthetized rats, and the effect of selected agents in attenuating or reversing these effects was studied. Infusion of L-NAME was used to validate the study model; it caused an immediate increase in tone and initiated phasic contractions indicating that the model was responsive to NO-mediated effects. Hemolink administration caused effects on intestinal motor function similar to those caused by L-NAME, including increases in basal tone and contraction amplitude. Rat whole blood caused none of these changes. The Hemolink-induced effects were less immediate in some animals compared to those observed after L-NAME. As well there was greater inter-animal variability on the effects. Hemolink administration also caused a mild increase in arterial blood pressure and a reciprocal decrease in heart rate in some animals. Co-administration of morphine, a common analgesic that has been reported to influence the motility of the GI tract; L-arginine, a substrate for NO synthesis; and glycopyrrolate, an anti-cholinergic agent, did not significantly modulate the Hemolink effects, whereas nitroglycerin, an NO donor; and nifedipine, a slow calcium-channel blocker, attenuated or reversed these effects.

Distribution of NADPH diaphorase-positive neurons in the enteric nervous system of the rabbit intestine

Nitric oxide (NO) has been proposed as an inhibitory transmitter in gastrointestinal muscle relaxation. We analyzed the distribution of nitric-oxide producing neurons in the rabbit intestine through nicotinamide-adenine-dinucleotide-phosphate-diaphorase histochemistry. By this reliable and convenient method, we visualized neuronal nitric-oxide-synthase, the enzyme responsible for nitric oxide generation, in the rabbit intestine. In the ileum and rectum, nitric-oxide-synthase-related diaphorase activity was present in the myenteric plexus ganglion cells, and in the nerve fibers in the internodal strand, secondary, and tertiary plexuses. These fibers were particularly abundant in the deep circular rather than in the outer longitudinal muscle layer. In the inner submucosal plexus, we found scarce labeled neurons. Labeled neural somata showed a range of sizes and shapes suggesting different functional roles. The present basic information is required to use the rabbit as an experimental animal in neurochemical NO enteric research.

Contribution of nitric oxide and substance P to nonadrenergic, noncholinergic transmission in the guinea pig ileum

1. The possible contribution of the nonadrenergic noncholinergic (NANC) transmitters nitric oxide (NO) and substance P (SP) to the contractility of guinea pig isolated ileum was studied by the responses of the longitudinal muscle to electrical field stimulation (0.8 msec, 40 V, 1-20 Hz, 20 sec) of the intrinsic nerves and by the presence and distribution of NADPH-diaphorase- and SP-positive nerve structures in the myenteric plexus. 2. The electrically elicited, tetrodotoxin (0.3 microM)-sensitive responses, in the presence of phentolamine (5 microM), propranol (5 microM), and atropine (3 microM) consisted of relaxation, followed by twitch and tonic contraction on which phasic contractions were superimposed. 3. NG-nitro-L-arginine (L-NNA; 0.1 mM or 0.5 mM), an inhibitor of NO synthesis abolished the relaxation. L-arginine (0.1 mM), a substrate for NO synthesis, but not D-arginine, restored it. L-NNA concentration dependently increased the twitch and tonic contractions. Sodium nitroprusside (1 microM or 10 (M), an exogenous donor of NO, was without effect on the electrically evoked responses. 4. AP 13.2 ACOH (AP; 0.1 microM or 10 microM), a blocker of SP receptors, frequency dependently inhibited or even prevented the twitch and tonic contractions. AP concentration-dependently increased the relaxation or reversed the responses to electrical stimulation into a deep relaxation. 5. The concentration-response curve for SP (1 nM-0.1 microM) was shifted to the right by AP, the EC50 values being 5.2 +/- 0.4 nM and 88.0 +/- 3.0 nM, respectively. The effects of SP were not altered by L-NNA (0.1 mM). 6. These findings, supported by morphological data about distribution of NADPH-diaphorase-positive nerve cell bodies and processes and SP-positive varicose fibers, suggest the contribution of NO and SP to NANC transmission. It appears that NO inhibits prejunctionally the SP transmission whereas SP counteracts the NO effect at the postjunctional level.

Transition of functional innervation in the developing porcine airway from nitrergic to catecholaminergic

1. We determined the distribution and chemical nature of the inhibitory neurotransmitter(s) to the airway smooth muscle (ASM) before and after birth. 2. Relaxation responses to electrical field stimulation (EFS) were studied in isovolumic bronchial segments from foetal (approximately 100/115 days gestation) and adult (25 kg) pigs, and in isovolumic tracheal segments from the foetus, and tracheal smooth muscle strips from the adult pig. Preparations were conditioned in low doses of atropine (10(-7) M) to reduce the effects of excitatory neurotransmission and then exposed to carbachol to produce submaximal muscle tone. Some studies were also carried out on bronchial segments from 4 week old pigs. 3. EFS (65 V, 2 ms, 5-20 Hz for 5 s) produced a TTX-sensitive relaxation in epithelium-intact and epithelium-denuded preparations. In foetal bronchial and tracheal preparations, EFS-induced relaxation was strongly inhibited by N(G)-nitro-L-arginine (L-NOARG, 10(-6) to 10(-4) M; P<0.01-0.001). However, in the adult, only relaxations of the trachea were inhibited by L-NOARG; bronchi were resistant to L-NOARG and also to N-nitro-L-arginine methyl ester (L-NAME, 10(-4) M). The inhibitory actions of L-NOARG (10(-6) to 10(-4) M) were substantially reversed by 10(-2) M L-arginine. Experiments with bronchial segments from 4 week old pigs showed partial inhibition of relaxations by L-NOARG. 4. The L-NOARG-insensitive relaxations recorded in the adult bronchus were blocked by propranolol (10(-6) M). 5. The onset of relaxation to EFS was more prompt and the rate of relaxation more rapid in foetal bronchi than in adult bronchi (P<0.0005). Maximum relaxation and recovery times were the same. 6. Foetal and adult bronchi were relaxed by sodium nitroprusside (SNP) with similar sensitivity and maximum effect. The rate of relaxation to SNP was not different in the two ages. 7. In the absence of atropine and carbachol, excitatory cholinergic responses to EFS (65 V, 2 ms, 5 Hz for 20 s) were not altered by L-NOARG (10(-4) M) or L-NAME (10(-4) M) in the adult bronchus but were modestly increased by L-NOARG in the foetal bronchus (P<0.01). 8. The tracheobronchial tree appears functionally innervated by nitrergic input to the smooth muscle before birth. However, at or after 4 weeks of age the inhibitory neural input to the bronchi is catecholaminergic, but it remains nitrergic in the trachea. There is also a weak nitrergic pre- or postsynaptic inhibition of the effects of cholinergic neurotransmission in the foetal bronchus but not in the adult.

Blockade of nitrergic neuroeffector transmission in guinea-pig colon by a selective inhibitor of soluble guanylyl cyclase

The role of soluble guanylyl cyclase in nitrergic inhibitory neuroeffector transmission was investigated in the longitudinal muscle from guinea-pig colon, by using an inhibitor of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ). In preparations precontracted with histamine, electrical field stimulation (EFS) or exogenous nitric oxide (NO) induced relaxations. The relaxation induced by NO-application was abolished by ODQ. Both ODQ and the NO-synthase inhibitor N omega-nitro-L-arginine (L-NOARG) partially inhibited the EFS-evoked relaxation to a similar extent. These effects were dose-dependent. The inhibition was more pronounced in the late phase of the EFS-induced relaxation. The inhibitory effect of ODQ on EFS-induced relaxation was not affected by additional application of L-NOARG. When NO-formation was blocked by L-NOARG, a subsequent addition of ODQ gave no further inhibition of the relaxation. These findings suggest that inhibitory non-adrenergic, non-cholinergic neurotransmission in guinea-pig colon is dependent on endogenous formation of NO, and that the NO-effect is exclusively mediated via the soluble guanylyl cyclase pathway. The existence of an NO-independent inhibitory transmission, which is not mediated through the cyclic GMP pathway, is also indicated. Furthermore, it is demonstrated that the NO/soluble guanylyl cyclase-independent transmission has an earlier onset as compared with the NO/soluble guanylyl cyclase-dependent pathway.

Nitrergic modulation of cholinergic responses in the opossum lower oesophageal sphincter

1. Electrical field stimulation (EFS) of the superfused lower oesophageal sphincter from opossum (Monodelphis domestica) elicited biphasic responses. The first phase (relaxation) was strictly dependent on the duration of the EFS. The second phase (contraction) started following termination of the EFS (< or = 15 Hz). EFS at frequencies above 15 Hz led only to contraction, which started immediately upon initiation of the stimulation. 2. In the presence of NG-nitro-L-arginine (L-NOARG; 0.1-300 microM), the relaxation phase was abolished and the contractile response started with the initiation of EFS (at all frequencies) and was greater in magnitude. The contractile response to EFS was completely blocked with scopolamine (10 microM). 3. Exogenous acetylcholine (1-100 microM) elicited concentration-dependent contractions of the sphincter in the presence of botulinum toxin. These contractions were abolished when EFS was applied during administration of acetylcholine. This inhibitory effect of EFS was completely reversed when the tissue was treated with L-NOARG (100 microM). 4. These results suggest that the cholinergic response in the opossum lower oesophageal sphincter is under nitrergic control.

Pattern of nonadrenergic, noncholinergic responses during short- or long-lasting electrical stimulation in guinea-pig ileum

1. The pattern of responses of longitudinally oriented guinea pig ileum organ bath preparations was studied during short- (1-5 sec) or long-lasting (20 sec) electrical field stimulation (EFS, 0.8 msec, 40 V, 1-20 Hz). 2. In the presence of phentolamine (5 microM), propranolol (5 microM), and atropine (3 microM), the EFS elicited nonadrenergic, noncholinergic (NANC), tetrodotoxin (0.3 microM)-sensitive responses. 3. The 1-sec EFS evoked relaxation. The response to 5-sec EFS consisted of relaxation followed by twitch, whereas relaxation, twitch and tonic contraction characterized the NANC response to 20-sec EFS. The maximum relaxation was observed at 10-Hz short- or long-lasting EFS. 4. Both N-G-nitro-L-arginine (L-NNA, 0.1-0.5 mM) and apamin (1-5 microM) concentration dependently inhibited the relaxation of the NANC response to 10-Hz 20-sec EFS. During L-NNA treatment, the twitch and the tonic contractions were increased. The inhibitory effect of L-NNA was reversed by L-arginine (0.1-0.5 mM) but not by D-arginine. Sodium nitroprusside (1-10 microM) was without effect. 5. AP 13.2 ACOH (0.1 microM), a blocker of Substance P receptors, inhibited the twitch and the tonic contractions. The contractions were decreased after desensitization of purinoceptors by ATP and in the presence of the GABA(A) receptor antagonist bicuculline (30 microM). 6. Depending on the EFS duration, a subsequent occurrence of relaxation and contractions characterized the NANC responses. It seems that relaxation is mediated by nitric oxide whereas Substance P and ATP are involved in the maintenance of the twitch and the tonic contractions. Nitric oxide appears to exert an inhibitory effect on the excitatory transmitters, whereas purinergic mechanism(s) could modulate the nitric oxide-dependent relaxation.

Neuromuscular facilitation and blockade induced by L-arginine and nitric oxide in the rat isolated diaphragm

1. L-Arginine (4.7-9.5 mM) induced an increase in the amplitude of muscular contraction (AMC) evoked by nerve stimulation of rat diaphragm preparations, but produced a reduction of the AMC evoked by direct stimulation of muscles previously treated with d-tubocurarine. The facilitatory dose of L-arginine was ineffective in changing the twitch tension evoked by retrograde injection of acetylcholine. 2. N omega-nitro-L-arginine (18 mM) antagonized the increase in AMC induced by L-arginine in preparations indirectly stimulated, and a similar effect was obtained against the depression induced by L-arginine in directly stimulated muscle preparations. D-Arginine (4.5-9.5 mM) was ineffective in changing the AMC evoked by direct or indirect stimulation of the diaphragm. 3. NO (8.6 mM) induced an increase of the AMC evoked by indirect stimulation of the muscle and was ineffective in changing the twitch tension evoked by retrograde injection of acetylcholine. NO (8.6 mM) produced an increase followed by a reduction of the AMC evoked by direct stimulation of muscles, but the muscular facilitatory effect induced by NO was smaller than the neuromuscular facilitatory effect. 4. These results indicate that NO increases the AMC when it interacts at the presynaptic level and decreases the AMC when it interacts at the postsynaptic level.

Nerve-induced release of nitric oxide in the rabbit gastrointestinal tract as measured by in vivo microdialysis

1. Nitric oxide (NO) has been suggested as a gastrointestinal neurotransmitter, mediating the gastric receptive relaxation and the relaxation in the peristaltic reflex. The aim of the present study was to measure nerve-induced NO formation in vivo in the gastrointestinal tract. 2. Formation of the nitric oxide oxidation products nitrite and nitrate during vagal nerve stimulation were measured in the anaesthetized rabbit. Microdialysis probes were inserted into the wall of the stomach and proximal colon, and nitrite and nitrate in dialysate measured by capillary electrophoresis. 3. During bilateral vagal nerve stimulation there was an increase in nitrite and nitrate formation at the level of the stomach and in nitrite formation at the level of the colon. This increase was inhibited by intravenous administration of the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME 30 mg kg-1). Furthermore, L-NAME significantly increased nerve-induced gastric and colonic contractions, as well as spontaneous colonic contractions. 4. In summary, we present a new methodological procedure for quantification of small changes in nitric oxide formation in vivo. This study provides evidence that nitric oxide is released in the stomach and colonic wall during vagal nerve activity, at concentrations able to cause inhibition of smooth muscle contractions in vivo.

Functional evidence for NO-synthase activation by substance P through a mechanism not involving classical tachykinin receptors in guinea-pig ileum in vitro

1. This study tested the hypothesis that a nitric oxide synthase (NOS) was activated in guinea-pig ileum in vitro in response to substance P (SP), and attempted to characterize the tachykinin receptor involved in this activation by the use of selective receptor agonists and antagonists. 2. Strips of guinea-pig ileum (8 x 2 mm) were superfused (Krebs, 37 degrees C, 2 ml min-1) with: (i) tachykinin receptor agonists: SP, GR 73,632 (NK1), GR 64,349 (NK2), senktide (NK3), and neuropeptide (NP) gamma; (ii) tachykinin receptor antagonists: CP 99,994 (NK1), SR 48,968 (NK2), SR 142,801 (NK3); (iii) nerve-related agents: carbachol (CCh), atropine, tetrodotoxin (TTX), hexamethonium; (iv) NOS inhibitors: N omega-nitro-L-arginine-methyl-ester (L-NAME), N omega-monomethyl-L-arginine (L-NMMA) and aminoguanidine (AG); (v) NO-related agents, L-arginine (L-Arg), D-arginine (D-Arg), sodium nitroprusside (NaNP) and methaemoglobin. Muscle contractility was recorded isometrically and quantified as integrated area of activity. 3. SP, tachykinin receptor agonists and NP gamma (10 pM to 10 microM), produced concentration-dependent contractions of ileal strips, with EC50s in the nanomolar range, and maximal responses (Emax) attained at 0.1 microM for SP and 1 microM for the other agonists. The Emax response to SP equalled that to KCl (60 mM) taken as a 100% control (99.3% [93.0-105.7]; mean and 95% CI; n = 12); a comparable Emax contraction was obtained with the other tachykinin receptor agonists (1 microM) as well as with CCh (1 microM). 4. Under baseline conditions, L-NAME (1 microM), L-NMMA (1 microM) and AG (1 microM), failed to contract the muscle strip. In contrast, when superfused for 3 min, 10 min after SP (0.1 microM), they induced a transient contraction of the strip (e.g. for 1 microM L-NAME: 50 to 70 s duration; amplitude 73 +/- 12%, n = 24). 5. The NOS inhibitor-induced contractile response was not obtained after KCl (60 mM), GR 73,632, GR 64,349, senktide or CCh (all up to 1 microM). In contrast, this contractile response was obtained after NP gamma (1 microM). 6. Blockade of tachykinin NK1, NK2 and NK3 receptors by continuous superfusion of CP 99,994, SR 48,968 and SR 142,801 (1 microM) respectively, starting 5 min before SP, did not modify the response to L-NAME, superfused 10 min after SP (0.1 microM). The contractile response to L-NAME (1 microM) was blocked by atropine (1 microM), superfused either before or after SP. In contrast, it persisted after TTX or hexamethonium (1 microM) superfused in the same conditions. 7. The amplitude of NOS inhibitor-induced contraction (1 microM) was dependent on the concentration of priming SP (1 pM to 1 microM). In contrast, the contractile response to NOS inhibitors (1 nM to 10 microM) of the ileum strip primed with SP (0.1 microM) was not concentration-related. 8. L-NAME-induced contraction was prevented by continuous superfusion of L-Arg (1 microM), but not D-Arg (1 microM). In addition, the NO donor, sodium nitroprusside (1 microM) and the NO scavenger, methaemoglobin (10 micrograms ml-1), both prevented the contractile response to L-NAME. 9. In summary, SP and to a lesser extent NP gamma, exert a permissive action allowing contractile stimulating effects of L-NAME, L-NMMA and AG, in guinea-pig ileum in vitro, by a mechanism which apparently does not involve tachykinin NK1, NK2 and NK3 receptors. This action is likely to result from the activation of a NO-synthase by SP in the vicinity of intestinal myocytes. Thus, L-NAME, L-NMMA or AG, by blocking this SP-induced NO production, unveiled a smooth muscle contraction which involves a cholinoceptor (atropine-sensitive) mechanism.

Nitric oxide-related neural components in the rat small intestine after transplantation

The changes in nitric oxide (NO)-related neural components in the transplanted small intestine are unknown. In this study, the NO neural component was examined using electrophysiological and NADPH-diaphorase histochemical techniques in a rat small bowel transplantation model. Syngeneic total small bowel transplantation was performed in 26 male Lewis rats using microsurgical techniques. The rats were divided into four groups: an untreated control group and animals at 1 (G1), 2 (G2), and 4 (G4) weeks after transplantation. Jejunal strips were mounted in a superfusion apparatus for examination. In the presence of atropine and guanethidine, the effect of the NO synthesis inhibitor L-NG-nitro-arginine (L-NNA, 100 microM) relaxation mediated by the nonadrenergic, noncholinergic (NANC) neural system was assessed following electrical stimulation at 2Hz. The inhibitory effect of L-NNA on relaxation was taken as an indicator of NO production. The percentage of inhibition in the control group, and in G1, G2, and G4 was 43.30% +/- 6.08% (mean +/- SE), 42.10% +/- 6.69%, 43.62 +/- 10.00%, and 52.46% +/- 6.00%, respectively. Inhibition in G4 was significantly higher than in the other groups (P < 0.01). The percentage of NADPH-diaphorase-positive fibers in the control group, G1, G2, and G4 was 25.06% +/- 4.70% (mean +/- SE), 26.27% +/- 2.17%, 24.73% +/- 2.87%, and 30.76% +/- 3.19%, respectively. Again, G4 showed a significantly higher level than the other groups (P < 0.01). We conclude that increased NO production may play a significant role in maintaining the intrinsic nervous system of the small intestine after transplantation.

A nitric oxide synthase activity is involved in the modulation of acetylcholine release in Aplysia ganglion neurons: a histological, voltammetric and electrophysiological study

The role of nitric oxide or related molecules as neuromodulators was investigated in the buccal and the abdominal ganglia of the mollusc Aplysia californica. In a first step we showed that reduced nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry and specific nitric oxide synthase immunohistochemistry labelled the same neurons and fibres in both ganglia, pointing to the presence of a neuronal nitric oxide synthase. In a second step, we performed voltammetric detection of nitric oxide-related molecules using a microcarbon electrode in a reduction mode. A peak identified as N-nitroso-L-arginine was detected at -1.66 V in both ganglia. The identification of this compound as a product of endogenous nitric oxide synthase activity was reinforced by the fact that its peak amplitude was decreased in the presence of NG-monomethyl-L-arginine, an inhibitor of nitric oxide synthase, and increased with its substrate, L-arginine. An additional proof of a nitric oxide synthase activity was the detection of nitrites and nitrates in high concentrations (millimolar range) by capillary electrophoresis. We also showed that these nitric oxide-related molecules modulated acetylcholine release at two identified synapses in these ganglia. L-Arginine decreased acetylcholine release at the inhibitory synapse (buccal ganglion), whereas it increased acetylcholine release at the excitatory synapse (abdominal ganglion). The nitric oxide synthase inhibitors, N omega-nitro-L-arginine and NG-monomethyl-L-arginine, had opposite effects. Moreover, the exogenous nitric oxide donor, 3-morpholinosydnonimine hydrochloride mimicked the effects of L-arginine on both inhibitory and excitatory cholinergic synapses. The identification of two cholinergic synapses where nitric oxide affects acetylcholine release in opposite ways provides a useful tool to study the cellular mechanisms through which nitric oxide-related molecules modulate transmitter release.

Nitric oxide: an overview

Nitric oxide (NO), a paracrine-acting gas enzymatically synthesized from L-arginine, is a unique biologic mediator that has been implicated in a myriad of physiologic and pathophysiologic states. It is an important regulator of vascular tone and may be the mediator of the hemodynamic changes involved in sepsis and cirrhosis. In addition, there is increasing evidence that NO is involved in coagulation, immune function, inhibitory innervation of the gastrointestinal tract, protection of gastrointestinal mucosa, and the hepatotoxicity of cirrhosis. It has already been speculated that NO may represent a point of control or intervention in a number of disease states. The purpose of this paper is to provide the surgeon with a broad overview of the scientific and clinical aspects of this important molecule.

Conditions required for the measurement of nitric oxide synthase activity in a myenteric plexus/smooth muscle preparation from the rat ileum

Nitric oxide synthase (NOS) activity was measured, by the conversion of arginine to citrulline, in a preparation from the rat ileum consisting of the myenteric plexus and smooth muscle layers. A variety of incubating media were used in order to establish the optimal conditions required for the assay. NOS activity was present in the soluble fraction and was Ca(2+)- and calmodulin-dependent, characteristic of neuronal NOS. Exogenous Ca2+ was required for activity to be detectable but NOS activity progressively decreased with Ca2+ concentrations above 1.25 mM. Activity varied with arginine concentration, reaching saturation at 6 microM, and required the addition of the co-substrate NADPH. Endogenous levels of co-factors in the crude soluble fraction were not sufficient to maintain NOS activity. Omission of flavin adenine dinucleotide and tetrahydrobiopterin from the incubation medium reduced activity by 90%, and both co-factors had to be present for maximal activity to occur. These results emphasize the need to control assay conditions when measuring NOS activity in crude preparations from peripheral tissue.

Effects of ginsenosides on vasodilator nerve actions in the rat perfused mesentery are mediated by nitric oxide

This study was designed to explore the effect of ginsenosides, saponins from Panax ginseng, on the vasodilator nerve actions in the rat perfused mesentery and the mechanism of this effect. In the rat perfusion mesentery, when adrenergic nerves were blocked by guanethidine (5 x 10(-6) M) and vascular muscle tone was increased with methoxamine (5 x 10(-6)-10(-5) M), transmural field stimulation produced a frequency-dependent vasodilator response, which is due to the release of calcitonin gene-related peptide; ginsenosides significantly suppressed this vasodilator response in a concentration-dependent manner (3-30 micrograms mL-1). After pretreatment with saponin (50 micrograms mL-1, 3 min) to damage endothelial cells, this suppressing effect of ginsenosides was unaltered. However, the effect was abolished by N omega-nitro-L-arginine methyl ester (L-NAME) (10(-4) M), an inhibitor of nitric oxide synthesis and addition of L-arginine (3 x 10(-4) M) restored this suppressing effect. Methylene blue (10(-5) M), an inhibitor of guanylate cyclase, also abolished the suppressing effect of ginsenosides. However, ginsenosides did not alter the relaxation responses caused by exogenous calcitonin gene-related peptide administration. We conclude that ginsenosides can produce an inhibitory effect on the vasodilator response prejunctionally in the rat perfused mesentery and that this effect of ginsenosides may be mediated by nitric oxide released from non-adrenergic, non-cholinergic nerves.

Nitric oxide-based possibilities for pharmacotherapy

The goal of nitric oxide (NO) based pharmacotherapy is to reach proper homeostasis of NO metabolism in the target tissue where endogenous production of NO is either too weak or excessively increased. In addition to the classic NO-based therapy of cardiovascular conditions with nitrates, a variety of new therapeutic possibilities have emerged including sexual disorders, gastrointestinal system, immunology, tumour growth regulation and respiratory disorders. NO levels of target tissues can be affected directly by NO donors, or indirectly by increasing the level of L-arginine, a substrate of nitric oxide synthase (NOS). While increased production of NO by induceable NO (iNOS) by, for example, cytokines does not at present seem therapeutically meaningful, increased NO production by constitutive NOS (cNOS) may be involved in the beneficial effects of ACE inhibitors or oestrogens. NO production may be pharmacologically decreased by inhibition of expression of iNOS by glucocorticoids while both cNOS and iNOS derived NO production is inhibited by administration of false substrates, for example L-NAME. Additionally, the respiratory system and related vessels can be reached directly and more selectively by inhalation of pure NO gas. Possible problems in administering NO and perhaps some NO-donors include the toxic nature of the compound itself whereby vital enzyme systems may be inhibited and tissue damaging radicals formed. Future prospects of NO-based pharmacotherapy may feature selective ligands to different NOS isoforms and tissue selective donors that release NO in a controlled fashion.

Vagal control of nitric oxide and vasoactive intestinal polypeptide release in the regulation of gastric relaxation in rat

1. Gastric motility and neurotransmitter release in response to vagal stimulation were studied using a vascularly isolated perfused rat stomach. Gastric motor responses were recorded by a strain gauge force transducer implanted on the proximal stomach. 2. Electrical stimulation of vagal trunk (0.5-20 Hz) produced a triphasic response which was composed of a rapid transient relaxation (first phase) followed by a phasic contraction (second phase) and a delayed prolonged relaxation (third phase). Maximum responses of the first, second and third phase were observed at 2.5, 5 and 10 Hz, respectively. Intra-arterial infusion of tetrodotoxin (0.1 microM) or hexamethonium (100 microM) completely abolished the triphasic response. 3. The nitric oxide (NO) biosynthesis inhibitor NG-nitro-L-arginine (L-NNA; 100 microM) significantly antagonized the rapid relaxation but had no effect on the delayed relaxation, while vasoactive intestinal polypeptide (VIP) antagonist (1 microM) significantly reduced the delayed relaxation without affecting the rapid relaxation. 4. In response to vagal stimulation, NO production ([3H]citrulline formation in gastric tissue preloaded with [3H]arginine) was maximum at 2.5 Hz, whereas VIP release into the venous effluent was largest at 10 Hz. Hexamethonium abolished vagal-stimulated NO production and VIP release. L-NNA had no effect on VIP release in response to vagal stimulation. 5. The nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperizinium (DMPP; 100 microM) also caused a triphasic response similar to that observed with vagal stimulation and produced a significant increase in VIP and NO formation. DMPP-evoked VIP release was not affected by L-NNA. Similarly, DMPP-evoked NO production was not antagonized by VIP antagonist. 6. These results suggest that vagus nerve stimulation evokes NO and VIP release via nicotinic synapses which cause different modes of relaxation of the stomach. There is no interaction between NO and VIP release in response to vagal stimulation.

Co-existence of nitrergic, peptidergic and acetylcholine esterase-positive nerves in the pig lower urinary tract

The distribution of NO synthase (NOS) immunoreactive nerves and the possible co-existence with other neurotransmitters were investigated in the pig lower urinary tract. NOS immunoreactive nerves were found in the muscle layer, in the lamina propria and around blood vessels. The density of NOS immunoreactive nerves was more prominent in the trigone and urethra than in the detrusor. All parts of the lower urinary tract were supplied by numerous acetylcholine esterase (AChE) positive nerves. The number of adrenergic nerves in the trigone and urethra was moderate to rich, whereas only very few adrenergic nerves were demonstrated in the detrusor. A low to moderate number of nerve fibres containing neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP) were observed in the trigone and urethra, while very few were found in the detrusor. A small number of nerves, confined to the trigone and urethra, were stained for calcitonin-gene-related peptide, somatostatin and leu-enkephalin. Nerve fibres exhibiting immunoreactivity to bombesin/gastrin releasing peptide, gastrin/cholecystokinin, substance P or neurokinin A were virtually absent. Co-localization studies revealed that some NOS-immunoreactive nerves also stained for NPY, VIP or AChE. The present study shows that nitrergic nerves are present in the pig lower urinary tract in a density lower than the cholinergic, but higher than any of the studied peptidergic nerves. Coinciding localization of NOS-positive nerves with nerves expressing AChE, VIP and NPY suggests that NO may have a role as a messenger in the lower urinary tract directly and by interaction with other transmitters.

Distribution pattern, neurochemical features and projections of nitrergic neurons in the pig small intestine

The presence and topographical distribution of nitrergic neurons in the enteric nervous system (ENS) of the pig small intestine have been investigated by means of nitric oxide synthase (NOS) immunocytochemistry and nicotinamide dinucleotide phosphate diaphorase (NADPHd) histochemistry. Both techniques yielded similar results, thus confirming that within the pig ENS the neuronal isoform of NOS corresponds to NADPHd. Intrinsic nitrergic neurons were not confined to the myenteric plexus; considerable numbers were also present in the outer submucous plexus. In the inner submucous plexus, NOS immunoreactivity or NADPHd staining was restricted to a few nerve fibres and nerve cell bodies. The nitrergic neurons displayed a wide variety in size and shape, but could all be characterized as being multidendritic uniaxonal. Nerve lesion experiments showed that the majority of the myenteric nitrergic neurons project in an anal direction. Evidence is at hand to show that a substantial proportion of these neurons contribute to the dense nitrergic innervation of the tertiary plexus and the circular smooth muscle layer. Some of the nitrergic neurons of the outer submucous plexus were equally found to send their axons towards the circular muscle layer. In some of the nitrergic enteric neurons, VIP, neuropeptide Y, galanin or protein 10 occurred colocalized, but not calbindin or serotonin. The present findings provide morphological evidence for the presence of NOS in a proportion of the enteric neurons in the small intestine of a large omnivorous mammal, i.e. the pig. The topographical features of the staining patterns of NOS and NADPHd are in accord with the results of neuropharmacological studies and argue for the existence of distinct nitrergic subpopulations acting either as interneurons or as motor neurons.

Nitric oxide-like activity in guinea pig colon as determined by effector responses, bioassay and chemiluminescence analysis

The role of nerve-induced release of nitric oxide (NO) as a modulator of neuroeffector transmission was studied in the longitudinal muscle of the guinea pig colon. The biological activity of a vascular relaxing factor released by nerve stimulation was examined in a bioassay cascade system. Furthermore, biochemical measurements of nerve-induced release of the NO metabolite nitrite (NO2-) were made with a chemiluminescence technique. Transmural nerve stimulation elicited contractile responses that were partly blocked by atropine and further inhibited after additional application of the tachykinin receptor antagonist CP-96, 345. The NO-synthase inhibitor N omega-nitro-L-arginine (NOARG) enhanced the nerve-induced contractions and concomitantly increased the basal degree of contraction ('tone'). The relaxations obtained by nerve stimulation after treatment with atropine and histamine were inhibited by NOARG. Electrical stimulation of the guinea pig colon released a non-adrenergic non-cholinergic (NANC) vascular relaxing factor into the tissue superfusate. The half-life of this factor down the cascade was the same as that observed with exogenous application of NO NOARG and tetrodotoxin (TTX) inhibited the release of the relaxing factor. During transmural nerve stimulation there was a significant increase in NO/NO2- release. This increase was inhibited by TTX and N omega-nitro-L-arginine methyl ester (L-NAME). In conclusion, pharmacological analysis as well as bioassay and biochemical measurements suggest that NO is released during nerve stimulation in the guinea pig colon, where it mediates smooth muscle relaxation.

Noradrenergic-nitrergic interactions in the rat anococcygeus muscle: evidence for postjunctional modulation by nitric oxide

1. The distribution of NADPH-diaphorase positive and catecholamine-containing nerve structures, and functional noradrenergic-nitrergic interactions, were studied in the rat anococcygeus muscle. 2. The morphological findings demonstrated NADPH-diaphorase positive neurons mostly as aggregates in intramural ganglia, nerve tracts and few single nerve fibres forming plexus-like structures. 3. The nitric oxide synthase inhibitor NG-nitro-L-arginine (L-NOARG) inhibited concentration-dependently the nitrergic relaxation, an effect reversed by L-arginine. The drug had dual effects on noradrenergic contractile responses: at lower concentrations (0.1-10 microM) it decreased the amplitude of contractions and this was not affected by L-arginine; higher concentrations (50-500 microM) potentiated the contractions, an effect that was prevented by L-arginine. 4. The electron acceptor, nitro blue tetrazolium (NBT) produced a rapid inhibition of the noradrenergic contractile responses (EC50 0.178 +/- 0.041 microM). The drug decreased the tone of the preparations. However, it potentiated concentration-dependently the nitrergic relaxations. 5. NBT (1 microM) had no significant effect on the relaxations induced by exogenously applied nitric oxide (NO)-donor sodium nitroprusside (SNP, 0.01-50 microM). However, the effect of NBT (0.1-10 microM) on the electrically induced relaxation was significantly decreased by L-NOARG (10 and 50 microM). The inhibition was of a non-competitive type. 6. Neither L-NOARG (100 microM) nor NBT (1 microM) had any effect on the spontaneous or electrically-induced release of 3H-radioactivity from the tissues preincubated in [3H]-noradrenaline. 7. It is concluded that L-arginine-NO pathway can modulate noradrenergic transmission in the rat anococcygeus muscle at postjunctional, but not prejunctional site(s).

Nitric oxide synthase in the rat carotid body and carotid sinus

The participation of nitric oxide synthase (NOS) in the innervation of the rat carotid body and carotid sinus was investigated by means of NADPH-diaphorase histochemistry and NOS immunohistochemistry using antisera raised against purified neuronal NOS and a synthetic tridecapeptide. NOS was detected in 23% of neurons at the periphery of the carotid bodies. Some negative neurons were surrounded by NOS-positive terminals. NOS-containing varicose nerve fibres innervated the arterial vascular bed and, to a lesser extent, the islands of glomus cells. These fibres persisted after transection of the carotid sinus nerve and are probably derived from intrinsic neurons. Large NOS-positive axonal swellings in the wall of the carotid sinus were absent after transection of the sinus nerve, indicating their sensory origin. The results suggest a neuronal nitrergic control of blood flow, neuronal activity and chemoreception in the carotid body, and an intrinsic role of NO in the process of arterial baroreception.

Neurogenic inhibition of duodenal and jejunal motility in the anaesthetized rat

Duodenal or jejunal motility (monitored as pressure changes in a saline-perfused intraluminal catheter) was studied in anaesthetized rats, vagotomized and pretreated with adrenergic blocking agents. In the duodenum (but not the jejunum), atropine or the selective muscarinic M1 and M3 receptor antagonists, pirenzepine and 4-diphenyl-acetoxy-N-methylpiperidine (4-DAMP), respectively, augmented the spontaneous contractile activity. This effect could be abolished either by nicotinic ganglionic receptor antagonism with hexamethonium, or with morphine. Moreover, blockade of the synthesis of nitric oxide by N omega-nitro-L-arginine elicited hypermotility both in the duodenum and the jejunum, and also this response was abolished by hexamethonium. It is proposed from the present results that the rat small is controlled by non-adrenergic, non-cholinergic inhibitory as well as excitatory motor neurons. The latter motor neurons seem to be modulated by muscarinic, nitroxergic or opioidergic mechanisms.

Increase by NO synthase inhibitors of acetylcholine release from guinea-pig myenteric plexus

The effects of nitric oxide (NO) synthase inhibitors on the electrically evoked release of [3H]acetylcholine were studied in guinea-pig myenteric plexus preparations preincubated with [3H]choline. NG-monomethyl-L-arginine (EC50 5.3 mumol l-1) and NG-nitro-L-arginine (EC50 1.3 mumol l-1) concentration-dependently increased the evoked release of [3H]acetylcholine without affecting the basal outflow. The facilitatory effect of NG-mono-methyl-L-arginine was prevented by L-arginine but not by D-arginine. The results suggest that endogenous NO inhibits the depolarisation-evoked release of acetylcholine.

Regional differences in nitric oxide-mediated relaxation of the rabbit sphincter of Oddi

We studied the role of the L-arginine-nitric oxide (NO) pathway in non-adrenergic, non-cholinergic (NANC) relaxation of the rabbit sphincter of Oddi by recording changes in isometric tension in response to electrical field stimulation in two series of experiments. In a first set of experiments, biliary sphincters of Oddi removed from New Zealand white rabbits were placed horizontally in an organ bath containing oxygenized, buffered (pH 7.4) Krebs solution. Contractile responses of the whole sphincter to field stimulation were determined. In the second set of experiments, sphincter of Oddi was divided into two parts and the effects of field stimulation were studied separately on areas close to the duodenal papilla (area I) and areas close to the common bile duct (area II). In the whole sphincter of Oddi, field stimulation induced an initial twitch-like contraction followed by relaxation proportional to the number of stimuli (3 and 10 stimuli at 20 Hz, 50 V, 0.1 ms). The magnitude of the contractile responses was considerably reduced by 1 microM atropine, phentolamine and oxprenolol (NANC solution). Field stimulation produced dose-dependent contractions of both segments of sphincter of Oddi in response to the same protocol as used with whole sphincter of Oddi. However, preincubation with NANC solution produced monophasic relaxations in response to field stimulation in area I, whereas area II preparations such as the whole sphincter of Oddi responded with contractions followed by minimal relaxations. Field stimulation failed to induce either contractions or relaxations in the presence of 1 microM tetrodotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial nitric oxide and cardiovascular disease

The vascular endothelium is the site of formation of several powerful mediators. One of these is NO, a chemically unstable radical formed by enzymatic conversion of L-arginine in the presence of molecular oxygen. NO elicits relaxation of VSMC by activating cytosolic guanylate cyclase. NO also counteracts platelet adhesion and aggregation. The biological actions of NO make it a key substance in the endogenous defense against vascular occlusion and thrombosis. The basal formation of NO maintains a moderate but significant vasodilation in the systemic resistance vessels and counteracts platelet activity. When blood flow in conduit arteries is increased there is an augmented endothelial formation of NO, eliciting flow-dependent vasodilation. Beside this, several vasodilators (acetylcholine, bradykinin, histamine, substance P) operate by stimulating endothelial NO formation. On the other hand, drugs like nitroglycerin and papaverine operate independently of the vascular endothelium. Vasodilator mechanisms, physiological as well as pharmacological, may therefore be characterized as endothelium-dependent (i.e. NO-mediated), or endothelium-independent (i.e. not mediated by NO). Physiologically, mixed mechanisms occur. Failure of the vascular endothelium to elicit NO-mediated vasodilatation may be due to decreased formation, increased degradation, decreased sensitivity to the NO formed, or a mixture of these factors. Irrespective of the mechanism behind, this is referred to as endothelial dysfunction. Endothelial dysfunction occurs in several cardiovascular settings, like atherosclerosis, hypercholesterolaemia, diabetes, and essential hypertension. Endothelial dysfunction leads to an impaired tissue perfusion, increased local vascular resistance, decreased defense against thrombus formation, and possibly also decreased defense against hypertrophy of the VSMC in the vessel wall media. In patients with CHD, endothelial dysfunction leads to an impaired coronary flow response to physical and mental stress, and to promotion of platelet adherence and aggregability. Endothelial dysfunction is thereby a probable aggravating factor in the atherosclerotic process, adding a functional component on top of the structural lesions characterizing this disease. A particular form of endothelial dysfunction, limited to the arterial resistance vessels, may explain the symptoms and clinical characteristics of microvascular angina. In patients with essential hypertension, endothelial dysfunction prevails, adding a functional component to the structural factors also in this disease. Hitherto, the only therapeutic tools available to restore endothelial dysfunction appear to be restriction of the dietary intake of lipids, possibly reinforced with intake of antioxidants like fish oil and vitamin E. However, large clinical trials to confirm the efficacy of such therapy in reversing endothelial dysfunction have not been conducted. In the future, more directly acting therapeutic regimens, aimed at supporting or substituting the endogenous formation of NO, are likely to appear as well.

Nitric oxide modulates the calcitonin gene-related peptide-induced relaxation in guinea pig gallbladder strips in vitro

Calcitonin gene-related peptide (CGRP) induces a dose-dependent relaxation in cholecystokinin-induced tension in guinea pig gallbladder strips in vitro. This effect seems to be modulated, in part, via activation of the nonadrenergic noncholinergic nervous system. Blockers of nitric oxide synthesis significantly reduced the CGRP effect. Increases in nitric oxide (NO) after treatment with CGRP suggested the involvement of NO in the observed relaxation.

Nitric oxide synthase-containing nerve fibers and neurons in the genital tract of the female mouse

Nitric oxide (NO) is generated intracellularly from L-arginine by the action of the enzyme nitric oxide synthase (NOS). The present investigation demonstrates immunoreactivity against NOS and nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase activity in nerve cells and fibers of the reproductive system of the female mouse. The density of nerve fibers staining for NOS varied among different genital organs. The ovary and Fallopian tube were devoid of NOS-positive nerves. The uterine horns received sparse innervation by NOS-containing nerve fibers. The most abundant NOergic innervation was found in the uterine cervix and vagina, where the nerve fibers ran parallel to the smooth muscle bundles and beneath the epithelium; they also accompanied intramural blood vessels. The vaginal muscular wall contained single or groups of NOS-reactive nerve cells. Clusters of NOS-containing neurons were located in Frankenhäuser's ganglion at the cervico-vaginal junction. NO may therefore act as a transmitter in the nervous control of the female reproductive tract.

Role of nitric oxide in the peristalsis in the isolated guinea-pig ileum

The role of nitric oxide in peristalsis was studied by using NG-nitro-L-arginine, an inhibitor of the biosynthesis of nitric oxide, in the isolated guinea-pig ileum. Constant peristalsis was cyclically induced by distending the ileal wall with intraluminal solution infused at a constant rate. NG-Nitro-L-arginine (10(-6) to 10(-4) M) dose dependently increased the frequency of peristalsis. This effect of NG-nitro-L-arginine was almost hindered by pretreating the ileum with 10(-3) M L-arginine, but not D-arginine. Nitroprusside (5 X 10(-7) M) reversed the frequency increase. In the presence of NG-nitro-L-arginine, peristaltic propulsion occurred at a smaller distension of the ileal wall and the ileum constricted to a smaller diameter at the completion of propulsion. The rate of shortening of longitudinal muscle during distension was raised by NG-nitro-L-arginine, although the peak magnitudes were not changed. Consistent with these effects of NG-nitro-L-arginine on peristalsis, NG-nitro-L-arginine at 10(-5) M increased the contractions of circular muscle in response to electrical field stimulation, but not those of longitudinal muscle. These results suggest that endogenous nitric oxide modulates peristalsis by limiting the contractile activity of the circular muscle of the guinea-pig ileum.